Vapor barriers

Q. We have a basement slab that was heaving, possibly due to poor drainage or expanding soil. The slab was removed and a new one poured.  It is a finished basement and we will be installing a pad and carpeting. Should the slab be sealed?

A. Concrete is porous and water vapor will eventually come through the slab. Many concrete sealers may prevent the intrusion of water but not water vapor. Sealers that stop all water tend to fail after a period of time because the water collects at the barrier/concrete surface. This water destroys the bond and the barrier fails.

ACI does not have a recommendation for this application. The most effective solutions, from external sources, are impermeable barriers that create an air space between the floor and barrier; or insulation directly on the surface of the floor with wood flooring on top. Both solutions are trying to create an equilibrium where moisture vapor is free to move but is no longer driven to the surface to collect by difference in pressure or temperature.

References: 360R-10; 302.1R-15

Topics in Concrete: Slab